Microwave-hydrothermal synthesis and hyperfine characterization of praseodymium-doped nanometric zirconia powders

Article abstract of DOI:10.1111/j.1551-2916.2005.00093.x

This work focuses on the microwave-assisted hydrothermal synthesis of praseodymium-doped zirconia and on the subsequent evaluation of the effect of synthesis conditions on powder properties. Pure and 10 mol% Pr-doped zirconia samples have been analyzed by X-ray diffraction (XRD) and the perturbed angular correlations hyperfine technique, which probes the nearest environments of zirconium ions. At atomic scale, as determined from perturbed angular correlation data, the XRD amorphous fraction of the as-obtained powders exhibits a tetragonal-like structure. The pure powder becomes partially stabilized and the doped powder is a substitutional solid solution of praseodymium in tetragonal zirconia.

Full text of DOI:10.1111/j.1551-2916.2005.00093.x

J. Am. Ceram. Soc., 88 [3] 633–638 (2005)
DOI: 10.1111/j.1551-2916.2005.00093.x
ournal
J
Microwave-Hydrothermal Synthesis and Hyperfine Characterization of
Praseodymium-Doped Nanometric Zirconia Powders
Federica Bondioli,w Cristina Leonelli, and Tiziano Manfredini
Dipartimento di Ingegneria dei Materiali e dell’Ambiente, Universita di Modena e Reggio Emilia, 41100 Modena, Italy
Anna Maria Ferrari
Dipartimento di Scienze e Metodi dell’Ingegneria, Universita di Modena e Reggio Emilia, 42100 Reggio Emilia, Italy
ÃÃ
ÃÃ
´
n Mario Rodrıguez
Maria Cristina Caracoche,* Patricia Claudia Rivas, and Agustı
´
´
Departmento de Fısica, Universidad Nacional de La Plata, 1900 La Plata, Argentina
This work focuses on the microwave-assisted hydrothermal syn-
thesis of praseodymium-doped zirconia and on the subsequent
evaluation of the effect of synthesis conditions on powder prop-
erties. Pure and 10 mol% Pr-doped zirconia samples have been
analyzed by X-ray diffraction (XRD) and the perturbed angular
correlations hyperfine technique, which probes the nearest envi-
ronments of zirconium ions. At atomic scale, as determined from
perturbed angular correlation data, the XRD amorphous frac-
tion of the as-obtained powders exhibits a tetragonal-like struc-
ture. The pure powder becomes partially stabilized and the
doped powder is a substitutional solid solution of praseodym-
ium in tetragonal zirconia.
ed that it is possible to obtain a metastable tetragonal t0-phase of
ZrO2 at room temperature under microwave-assisted hydrother-
mal conditions starting from ZrOCl2 1M and NaOH 0.5M.9
It is well known that zirconia is a typical polymorphic mate-
rial that may exist in monoclinic (m), tetragonal (t), and cubic
(c) crystalline forms at different temperatures.10 However, the
phase formation and the transformation behavior of hydrous-
zirconia powders are affected by various synthesis conditions or
processing parameters11 and, thus, may not always follow the
traditional phase transition route. Studies on new dopant agents
capable of producing zirconia-based powders with improved
physical and chemical properties are still of great interest. Dop-
ing of Me(III) oxides leads to both an increase in the oxygen
vacancy concentration and an enhancement of the oxygen-ion
conductivity, which enables to use these stabilized zirconia as an
electrolyte in the fuel cells. The purpose of the present work is to
report the synthesis of ultrafine Pr-doped ZrO2 powders under
microwave-assisted hydrothermal conditions and the effect of
the praseodymium ion on the zirconia phase stabilization. Pra-
seodymium dopant has been chosen because it is one of the
elements under investigation in the field of oxygen storage ma-
terials and it undergoes oxygen exchange at a lower temperature
than cerium oxide12 and its oxygen storage capacity is not
diminished by high sintering temperature.13
I. Introduction
HERE has been an increasing trend toward the use of finer
powders in ceramic processing. A growing demand for im-
T
proved performance of ceramics in load-bearing applications
and in functional application has raised the importance of chem-
ical purification and processing of the starting powders. Maxi-
mum control of a ceramic process begins with the starting
material and powder preparation. Consequently, numerous
chemical unconventional preparation methods that include
forced hydrolysis, urea-based homogeneous precipitation, co-
precipitation, hydrothermal synthesis, flux method, reaction in
microemulsion, and sol–gel synthesis,1 likely to lead to suitable
powders, have been extensively investigated.
The possibility of PrOx–ZrO2 solid solution formation has
been hypothesized, the phase diagram being unknown, on the
basis of the lanthania–zirconia phase diagram and by consider-
31
ing the ionic radii of Zr41 (0.87 A) and both Pr (1.126 A) and
˚
˚
Among the different methods, the hydrothermal crystalliza-
tion2–4 is an interesting process to directly prepare nanometer-
sized crystalline powders with reduced contamination and low
synthesis temperature. A late innovation to the hydrothermal
method, developed by Komarneni et al.,5–7 involves the intro-
duction of microwave during the hydrothermal synthesis to in-
crease the kinetics of crystallization by one to two orders of
magnitude and sometimes to lead to novel phases. As it has been
recently reported,8 the microwave application is an efficient way
to enhance powder crystallinity and decrease processing times.
In particular, in a previous work, the authors have demonstrat-
Pr41 (0.90 A). Previous studies of the authors14 have showed, in
˚
the 201–12001C thermal range, the solubility of praseodymium
ion in the tetragonal zirconia structure.
The study focuses on the preparation and characterization of
the pure and Pr-doped (Pr 5 10 mol%) zirconia powders. In
particular, in order to evaluate the metastable zirconia phases
present in the samples, the perturbed angular correlation (PAC)
spectroscopy has been used. The PAC method has proved to be
an efficient tool in the investigation of zirconia-based ceramics
at an atomic level,15–19 since it allows the determination of dif-
ferent atomic configurations around zirconium sites hardly re-
solvable by other techniques, which could help in explaining the
ceramic bulk properties and their thermal evolution. In fact, the
impurities of hafnium (1–5%) always existing in natural zirco-
nium, randomly distributed at substitutional zirconium sites,
constitute the hyperfine probes of the technique. The thermal
neutron irradiation of the samples activates some 180Hf isotopes
by transforming them in the 181Hf probes.
S.-I. Hirano—contributing editor
Manuscript No. 186893. Received June 19, 2002; approved January 26, 2004.
Supported by MURST-PRIN Contract, ‘‘Application of the microwave technology to
physical–chemical processing involving solids—1999–2000’’.
´ ´
Partially supported by Comision de Investigaciones Cientıficas de la Provincia de Bue-
nos Aires and CONICET, Argentina.
The method briefly consists in an inspection of the angular
correlation of two energetic photons emitted in succession by the
radioactive nucleus 181Hf through the 133–482 keV disintegra-
*Member of CICPBA, Argentina.
Member of CONICET, Argentina.
ÃÃ
wAuthor to whom correspondence should be addressed. e-mail: fedebond@unimo.it
633

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Journal of the American Ceramic Society—Bondioli et al.
Vol. 88, No. 3
tion g–g cascade. The comparison of the measured angular
correlation against that of the isolated probe, known from the
nuclear physics, allows to gather information about the electric
field gradients (EFGs) existing in the lattice where the nuclear
probes Hf(Zr) are immersed. The information is drawn from the
determination of the so-called quadrupole parameters that de-
scribe the EFG at the zirconium site, i.e. its intensity (through
the quadrupole frequency oQ), its departure from axial symme-
try (through the asymmetry parameter Z), and its degree of dis-
order due to the presence of impurities or defects in the lattice
(through the broadening or spread d of the EFG). All of them
can be derived from the fitting of the experimental spin rotation
curves obtained at the laboratory. In addition, due to the ex-
tremely localized nature of the technique, non-equivalent sites in
the lattice can be distinguished through their relative abundanc-
es f. Even small changes as single-atom relaxations, vacancies,
and point defects can be detected as a modification of the par-
ticular hyperfine fingerprint corresponding to each of the zir-
conia polymorphs. A good example of the PAC high sensitivity
is the find that both tetragonal15–18 and cubic19 yttria-doped
zirconia metastable phases were described by a two-component
hyperfine interaction involving zirconium surroundings, each
distorting the lattice structure to a different extent. In the case of
the tetragonal structure, one of the interactions describes the
regular, eight oxygen-coordinated zirconium sites, denoted here-
inafter as t-form on account of its similitude with the zirconium
surroundings composing the crystalline equilibrium tetragonal
phase. The other one describes the highly disordered, oxygen-
defective zirconium surroundings due to the existence of struc-
tural oxygen vacancies and/or defects derived from the prepa-
ration method. It exhibits a lower tetragonality and is denoted
as t0-form. As it has already been reported,18 the PAC patterns
of both metastable configurations around zirconium are quite
distinguishable and also different from those of the equilibrium
tetragonal phase. The hyperfine parameters characterizing the
various tetragonal atomic configurations are the following:
oQ 5 165 Mrad/s, ZD0.55, and d ꢀ 25% for metastable t0-
form; oQ 5 185 Mrad/s, ZD0.2, and dr2% for metastable t-
form and oQ 5 160 Mrad/s (at 11501C), Zr0.2, and dr2% for
the equilibrium t-phase.
Research Laboratories, Eindhoven, the Netherlands). The X-
ray diffraction (XRD) patterns were collected in a 2y range of
251–901 at room temperature, with a scanning rate of 0.0051/s
and a step size of 0.021. Lattice parameters of monoclinic [P21/
a]20 and tetragonal [P42/nmc]21 phases, as well as their quantity,
were determined by the combined Rietveld and reference inten-
sity ratio (RIR) methods using GSAS.22,23 A 10 wt% of corun-
dum (NIST SRM676) has been added to all samples as an
internal standard. The mixtures, ground in an agate mortar,
have been side loaded in an aluminum flat holder in order to
minimize the preferred orientation problems. Data have been
recorded in the 51–1401 2y range (step size 0.021 and 6 s counting
time for each step). The phase fractions extracted by the Riet-
veld–RIR. refinements have been rescaled on the basis of the
absolute weight of corundum originally added to the mixtures as
an internal standard, and therefore internally renormalized.
The average crystallite size was calculated using the Sherrer’s
formula from the width of the XRD lines.24 The thermal sta-
bility of the powders up to 12001C was evaluated by therm-
ogravimetric and differential thermal analysis (TG-DTA)
(Model 404, Netzsch, Selb, Germany) and a heating rate of
101/min. The sample morphology and microstructure were ex-
amined by transmission electron microscopy (TEM) (Model
EM400, Philips Research Laboratories). Specimens were pre-
pared by dispersing the as-obtained powder in distilled water
and then placing a drop of the suspension on a copper grid
coated with a transparent polymer and then dried. The grain size
distribution was determined from TEM images. The maximum
diameters of more than 100 particles in TEM micrographs were
measured. The surface area analysis was carried out on the as-
prepared powders by BET (Model Gemini 2360, Micromeritics
Instrument, Norcross, GA), using nitrogen as an adsorbate. The
grain size was also calculated using the specific surface area
data, by the equation:
6
f ¼
Sr
where f is the average diameter of a spherical particle, S is the
surface area of a powder, and r is the experimental density value
of powder measured by a He picnometer (Model Accupic 1330,
Micromeritics Instrument).
II. Experimental Procedure
Regarding the PAC experiments, the samples were encapsu-
lated in air at atmospheric pressure in 0.5 cm3 sealed quartz
tubes and then irradiated with a flux of about 1013 ꢁ neu-
trons ꢁ cm–2 ꢁ s during 24 h in order to achieve the desired activ-
ity (about 100 mCi) of 181Hf. Measurements were carried out
using two BaF2 detectors in a planar arrangement. Each one
lasted about 2 days and was performed in air at atmospheric
pressure at increasing temperatures up to 6001C for the pure
sample and up to 11501C for the doped one. A final measure-
ment at room temperature was taken for the doped sample.
Details of the setup, data acquisition, and data handling are
given elsewhere.16
(1) Sample Preparation
The microwave-assisted hydrothermal synthesis of ZrO2
powders was conducted starting from ZrOCl2 ꢁ 8H2O (0.5M)
aqueous solutions. Praseodymium was added as nitrate salt
(Pr(NO3)3) in order to prepare the doped samples. Both solu-
tions were neutralized with NaOH (1M) to pH 9. The sus-
pensions were then treated in a Teflon-lined vessel using a
microwave digestion system (Model MDS-2000, CEM, Mat-
thews, NC). This system uses 2.45 GHz microwaves and is con-
trolled by pressure. It can attain a maximum pressure of 14 atm.
The reaction vessel was connected to a pressure transducer that
monitors and controls the pressure during synthesis.
After preliminary tests, microwave–hydrothermal treatments
were conducted at 14 atm for 2 h. Time, pressure, and power
were computer-controlled. The power level was automatically
adjusted to the required pressure. After the synthesis reaction,
the pure and doped powders were filtered by centrifugation,
washed, and dried in an electric furnace at 1201C for 12 h. To
evaluate the presence of Na and Cl and thus the efficacy of the
washing step, the supernatant was analyzed by ICP spectros-
copy (Model Liberty 200, Variant, Sydney, Australia). In par-
ticular, the absence of alkali and chlorine ions in the last
washing water indicated the efficacy of the washing step.
III. Results
(1) Pure Zirconia (Zr0)
To investigate the nature of the as-obtained powder, the sample
was subjected to XRD analysis. Figure 1 clearly indicates that
the as-synthesized sample is partially stabilized: although pre-
dominantly monoclinic, peaks corresponding to the tetragonal
ZrO2 phases are also present. In spite of that fact that an un-
ambiguous determination between the possible tetragonal crys-
tal structures could not be made from XRD results solely since
their reflections overlap, the relative intensity of the monoclinic,
tetragonal, and amorphous phase has been calculated by Riet-
veld–RIR (see Table I). Table II reports the XRD-derived av-
erage crystallite size for the tetragonal and monoclinic structures
and the interplanar spacing d for the tetragonal phase using
the (111) main diffraction line. Also included is the grain size
(2) Powder Characterization
To investigate the structure and crystallinity of the samples, the
synthesized products were analyzed with a computer-assisted
X-ray (CuKa) powder diffractometer (Model PW3710, Philips

March 2005
Microwave-Hydrothermal Synthesis and Hyperfine Characterization
635
(a)
Ã
Fig. 1. X-ray diffraction pattern of as-obtained Zr0 sample. , m-ZrO2;
1, t-ZrO2.
Table I. Quantitative Analysis Results and Standard Rietveld
Agreement Factors Obtained for Zr0 and Zr10 Samples
20 nm
Samples
(b)
30
Phases
Zr0
Zr10
m-ZrO2
w
36.6 (4)
22.8 (1)
40.5 (2)
99.9
2.85
2.58
–
25
20
15
10
5
t-ZrO2
Amorphous
Total
37.7 (1)
62.2 (3)
99.9
3.46
2.72
Rwp (%)
Rp (%)
w2
1.98
wThe structures have tetragonal P42/nmc symmetry.
2.11
Table II. Structural and Physical Properties of Powders
0
Average
Average
d
111(nm)
3
5
7
9
11 13 15 17 19
nm
Sample
diameter (nm) BET
diameter (nm) XRD
XRD
Fig. 2. Transmission electron micrograph of Zr0 sample (a) and grain
size distribution obtained by image analysis (b).
Zr0
8.5
6.7
m-phase: 12
t-phase: 11
7.9
0.295
0.298
Zr10
XRD, X-ray diffraction.
(2) Doped Zirconia (Zr10)
As shown in Fig. 5, the as-obtained doped product is a finely
divided powder of quite stabilized tetragonal zirconia. Also in
this case an unambiguous determination of the crystal structure
cannot be made solely from XRD results. Moreover, the quan-
titative analysis performed by the Rietveld–RIR method shows
determined by BET. The differences between the average crys-
tallite size as determined by X-ray line broadening and TEM
image analysis (Fig. 2), and the average grain size obtained by
specific surface area analysis are evidence of light grain agglom-
eration.
In Fig. 3, the weight loss and the differential thermal analysis
curves up to 12001C are reported. The observed weight loss is
due, in addition to both chemically coordinated and physisorbed
water, to residual oxo-bridging and non-bridging structural hy-
droxyl group and it is a sign of the crystallization degree of the
powders. The DTA curve exhibits, at about 701C, the endother-
mic peak characteristic of water elimination and around 4201C,
a weak exothermic peak. The sharp endothermic peak near
12001C corresponds to the equilibrium transition m- to t-ZrO2,
whose reversibility is confirmed by the exothermic signal ob-
served near 9001C in the cooling curve.
Relative fractions of the different hyperfine components ob-
tained in the fitting procedure of the PAC spectra have been
plotted in Fig. 4. At room temperature the powder composition
is 68% of t0, 5% of t-phase, and 27% of a very disordered m-
phase, indicating that pure zirconia has partially stabilized in the
tetragonal phase. As the temperature increases, the t0 configu-
rations transform to both, the more regular and crystalline t-
phase, which achieves its maximum concentration at 4001C, and
the monoclinic phase, whose relative fraction shows a perma-
nent increase with temperature. At 6001C, the conversion of the
tetragonal polymorphs to monoclinic zirconia is manifest.
0
−2
20
15
10
5
COOLING
EXO
−4
0
HEATING
−6
−5
−10
−15
−20
−8
−10
0
400
800
1200
Temperature (°C)
Fig. 3. Thermogravimetric and differential thermal analysis curves of
Zr0 sample.

636
Journal of the American Ceramic Society—Bondioli et al.
Vol. 88, No. 3
(a)
20 nm
Fig. 4. Perturbed angular correlation relative fractions of Zr0 as a
function of temperature.
the presence of an amorphous X-ray phase (see Table I). The
shift of the tetragonal diffraction line toward lower angles as
compared with pure zirconia indicates that praseodymium dop-
ing causes an increase in the interplanar spacing (shown in Table
II) and consequently in the cell dimensions. Table II also reports
the average grain size determined by both BET values and XRD
pattern analysis. The obtained results match with TEM results
obtained on this sample, contained in Fig. 6, indicating well-
dispersed particles.
TG and DTA curves up to 12001C are reported in Fig. 7. The
TG behavior is very similar to Zr0 sample evidencing the un-
completed crystallization of the starting product. The DTA
curve shows, near 801C, the endothermic signal characteristic
of water elimination and at around 4801C, an exothermic signal.
At higher temperatures, the DTA curve indicates the thermal
stability of the synthesized powders. Nevertheless, a subtle base-
line change can be observed at 9001C on heating and cooling. A
new diffractogram (Fig. 8(a)) obtained after the DTA experi-
ment up to 12001C allows to distinguish the stabilized zirconia
polymorph. In fact, an inspection of region 721–761 (Fig. 8b)
reveals that the Pr addition stabilizes the tetragonal structure.25
In fact, the high-angle region pattern shows the splitting of the
(400) and (004) reflections characteristic of a tetragonal phase
(otherwise, the cubic phase only presents the (004) reflection in
this region).
(b)
35
30
25
20
15
10
5
0
3
5
7
9
11
nm
13
15
17
19
Fig. 6. Transmission electron micrograph of Zr10 sample (a) and grain
size distribution obtained by image analysis (b).
Figure 9 shows the thermal evolution of the relative fractions
of the hyperfine interactions fitted. In agreement with the XRD
data, PAC results indicate that the product obtained under the
microwave–hydrothermal synthesis is completely stabilized in
the tetragonal phase, indicating, in addition, that it is depicted
by the defective and disordered tetragonal t0-form. Up to 4001C,
only the t0-ZrO2 phase is present. At 6001C, a small amount of
the crystalline t-ZrO2 phase appears. At 8001C, an additional
unknown interaction, hereinafter called X, becomes necessary to
obtain a satisfactory fit. As temperature increases up to 11501C,
PAC results indicate that the t-phase and the configuration de-
scribed by the X-interaction grow at the expense of the t0-phase.
0
−2
12
10
8
EXO
COOLING
−4
6
−6
HEATING
4
−8
2
−10
0
0
400
800
1200
Temperature (°C)
Fig. 7. Thermogravimetric and differential thermal analysis curves of
Zr10 sample.
Fig. 5. X-ray diffraction pattern of as-obtained Zr10 sample. 1, t-ZrO2.

March 2005
Microwave-Hydrothermal Synthesis and Hyperfine Characterization
637
sample still has many OH (see the TG curve), probably in re-
sidual amorphous regions whose atomic structure is undetecta-
ble by XRD analysis. The exothermic peak present in the DTA
curve at 4201C must be considered as being a result of the crys-
tallization of the residual amorphous phase. This thermal
change, observed at 4801C for Zr10, confirms that the crystal-
lization temperature increases with the addition of praseodym-
ium,14 and it is correlated with a weight loss matching to the
thermal dehydroxylation pathway:
Zr4Oð8ꢂxÞðOHÞ2x ! ZrOð2ꢂy=2ÞðamorphousÞðOHÞy
! t-ZrO2ðcrystallineÞ
characteristic of precipitated samples.26
The atomic-scale PAC information allows reporting that the
starting powder is predominantly in the defective and disordered
tetragonal t0-phase. This fact justifies the assumption that the
structure of the amorphous phase is tetragonal-like, as it has
been suggested by other authors on the basis of X-ray radial
distribution27 and X-ray diffraction data.28 As temperature is
increased, the crystallization occurs and the t0-phase gradually
transforms into the more regular tetragonal structure and
monoclinic phase. While the former structure shows a drastic
increase of its relative fraction near the temperature of the DTA
exothermic peak, the latter grows continuously with tempera-
ture, indicating the irreversible degradation of the ceramic. Ac-
cordingly, at temperatures higher than 6001C, DTA reveals that
the powder evolves as ordinary pure zirconia.
Concerning the powder doped with 10% of praseodymium,
XRD indicates that the resulting microwave–hydrothermal
product is wholly stabilized. A comparison with XRD informa-
tion obtained on the undoped sample allows to state that this
difference is due to the presence of additional oxygen vacancies
created by the substitutional replacement of Zr cations by Pr
cations. This localization of the dopant cations is coherent with
the shift of the diffraction lines toward lower angles.
PAC characterization, in turn, evidenced that the stabilizat-
ion led to 100% of tetragonal zirconia in its t0-phase, this
defective structure now including, in addition, the oxygen-de-
fective atomic configurations produced for charge balance
inherent to substitutional Pr replacements. The thermal trans-
formation between the disordered t0- and the ordered t-phases
that occurred between 4001 and 6001C can be associated to hy-
droxyl removal. The second change, from t0-phase to X-form, is
observed at 8001C. Regarding the last interaction, based on the
subtle reversible DTA signal at 9001C and on results concerning
structural changes in stabilized zirconias, it has been assumed
that it corresponds to a metastable cubic structure. In fact,
Boulc’h et al.29 have reported that in some sesquioxide-ZrO2
nanostructured solid solutions, a step in the base-line reflecting
the second-order t-c transition was observed for dopant con-
centrations higher than 2.5 mol%. Moreover, Withers et al.30
have reported, for the same composition as ours, the existence of
a ‘‘defect fluorite-type’’ cubic solid solution in an equilibrium
situation at higher temperatures.
Fig. 8. X-ray diffraction pattern of Zr10 sample after differential ther-
mal analysis (DTA) analysis (a), selected 721–761 angular region after
DTA analysis (b) and X-ray diffraction pattern after perturbed angular
correlation measurements (c).
The cooling from 11501C to room temperature causes a partial
degradation of the sample, yielding 43% of m-ZrO2, 16% of t-
form, and 41% of t0-form. No X-form is present. A final XRD
experiment performed on the activated sample used for the PAC
experiment reproduced this result, as can be observed in
Fig. 8(c).
IV. Discussion
Pure zirconia synthesized by the microwave–hydrothermal
method become partially stabilized. Particle size obtained from
different techniques on this sample indicates that stabilization
has proceeded by a size effect. Moreover, TG/DTA information
and the quantitative results obtained by the Rietveld–RIR meth-
od suggests that the crystallization of the obtained powders is
not completed under the present synthesis conditions. Although
there might be OH-free domains of t-ZrO2, the as-obtained
Concerning the thermal stability of the doped phase, it can be
observed that the relevant amount of monoclinic phase deter-
mined by PAC after cooling from 11501C, also revealed by the
diffractogram of Fig. 8(c) obtained on the same active sample, is
not present in the diffractogram shown in Fig. 8(a). This con-
troversial result can be explained considering the different heat-
ing treatments inherent to both experiments. It is evident that
the time used for the DTA analysis, much shorter than the du-
ration of each PAC experiment, was not enough to degrade the
stabilized zirconia solid solution.
V. Conclusions
Microwave-assisted hydrothermal synthesis has proved to be
a very suitable environmentally friendly processing method for
the synthesis of praseodymium-doped (Pr 5 10 mol%) zirconia
Fig. 9. Perturbed angular correlation relative fractions of Zr10 as a
function of temperature.

638
Journal of the American Ceramic Society—Bondioli et al.
Vol. 88, No. 3
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stabilized tetragonal zirconia with relatively high crystallinity.
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in the starting products. In particular, the possibility of per-
forming the hyperfine experiments as a function of temperature
allowed an in situ investigation on the thermal pathway of the
microwave–hydrothermal-synthesized zirconia.
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the exothermic peak, present in both the investigated samples,
respectively, at 4201 and 4801C, is to attribute to the crystalli-
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scale, as determined by PAC, transforms from the defective and
disordered t0-ZrO2 into the more regular t-ZrO2. The predom-
inance of the defective t0-form over the whole thermal range in-
vestigated and the shift of the diffraction lines towards lower
angles are consistent with the localization of Pr ions at substi-
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able Tetragonal Forms in ZrO2–2.8% Y2O3 Ceramics,’’ J. Am. Ceram. Soc., 79,
´
nez, A. M.
´
´
´
831–36 (1996).
17R. Caruso, E. Benavı
´
dez, O. de Sanctis, M. C. Caracoche, P. C. Rivas, M.
Cervera, A. Caneiro, and A. Serquis, ‘‘Phase Structure and Thermal Evolution in
Coatings and Powders Obtained by the Sol–Gel Process: Part II. ZrO2–2.5 mole%
Y2O3,’’ J. Mater. Res., 12 [10] 2594–601 (1997).
18A. M. Rodrı
´
guez, M. C. Caracoche, P. C. Rivas, A. F. Pasquevich, and S. R.
Mintzer, ‘‘PAC Characterization of Nontransformable Tetragonal t0 Phase in Arc-
Melted Zirconia-2.8 mol% Yttria Ceramics,’’ J. Am. Ceram. Soc., 84 [1] 188–92
(2001).
Acknowledgments
The authors are grateful to Dr. Daniele Verucchi, who performed the ex-
perimental procedure and Dr. Paola Miselli for assistance in XRD diffraction
analysis.
The S&T cooperation 1999–2000 between Italy (Ministero degli Affari Esteri)
and Argentina (SETCIP-MAE) project (IT.12/99) is gratefully acknowledged.
19P. C. Rivas, M. C. Caracoche, J. A. Martı
nez, A. M. Rodrıguez, R. Caruso,
´ ´
N. Pellegri, and O. de Sanctis, ‘‘Phase Structure and Thermal Evolution in Coat-
ings and Powders Obtained by the Sol–Gel Process: Part I. ZrO2–11.3 mole%
Y2O3,’’ J. Mater. Res., 12 [2] 493–9 (1997).
20Powder Diffraction File, Card 37-1484, International Centre for Diffraction
Data, Newtowne Square, PA, 1995.
21Powder Diffraction File, Card 42-1164, International Centre for Diffraction
Data, Newtowne Square, PA, 1995.
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